Commercial sound systems for concerts are expected to obtain a quality substantially beyond home systems and are therefore extremely high priced. One problem which continues, despite the high price, is low frequency degradation and low frequency reverberation in confined spaces due to dispersion, especially in the bass waves which can directly result in low frequency feedback.
Bass waves are extremely inefficient and are subject to the greatest loss and dispersion in comparison to mid and high range frequencies.
A significant limitation of the current state of the art acoustic transducers has been their frequency dependent beamwidth. The beamwidth of compression drivers, as well as more conventional transducers, is a function of both the size of the vibrating element and the transducer size, in the case of conventional transducers or the exit dimension and the frequency of vibration, in the case of compression drivers. Compression drivers make use of an acoustic impedance matching device in the shape of a horn attached to the exit of the driver. This device partially controls the beamwidth as well as improves efficiency. Although this solution greatly improves efficiency, it only partially resolves the beamwidth frequency dependence. Other compression drivers attempt to reduce frequency dependence by resorting to very small compression driver exits, however this reduces transducer efficiency.
Recently, a transducer system appeared in the state of the art which controls beamwidth dependence through use of an enclosure which is shaped as the envelope of ellipsoids. The enclosure has radially oriented distinct focal points as well as a common focal point. Transducers placed at the distinct focal points will have their acoustic radiation focused at the common focal point and provided that the ellipsoids have essentially identical path lengths from distinct focal point to ellipsoid to common focal point, their acoustic energy will be coupled in phase. Further, the beamwidth of this device is wide and essentially frequency independent. The device, however, displays a radial interference pattern when the acoustic radiation of the transducers, located on the radially distributed distinct focal points, interact. Specifically, transducers on neighboring distinct focal points can destructively interfere with each other, thereby causing radial diffraction or combing in the acoustic radiation field.
Another transducer system which has appeared in the state of the art overcomes the radial interference problem by use of an acoustic reflective lens. The lens shape is produced when a section of an ellipse is revolved about a line which passes through one of the focal points of the ellipse at some finite angle with respect to the ellipse major axis. In one embodiment, the lens is shaped like that of a cone, the sides of which are concave and describe a continuum of sections of identical ellipses. One focal point, common to all the ellipses, is positioned directly above the apex of the cone while the other focal points of the ellipses describe a circle around the base of the cone. A transducer placed at the common focal point, facing the apex, will have its radiation reflected from the cone surface and, owing to the elliptical shape of the sides of the cones, focused coherently onto the focal circle referred to above. The focal circle will appear as a virtual source of coherent acoustical radiation which, due to the symmetry of the cone, will emanate equally from the base of the cone. The beamwidth of acoustical radiation perpendicular to the base of the cone is controlled by the elliptical shape of the sides of the cone. Two such cones, with accompanying transducers, may be placed base-to-base in order to produce a radiation pattern who horizontal beamwidth is 360 degrees and who vertical beamwidth is frequency invariant.
A limitation of this reflective lens exists that is due to the use of the elliptical geometry. This geometry constrains radiation to emanate, or appear to emanate, from the common focal point of the acoustic lens in order to be coherently focused onto the focal circle. Currently available transducers do not produce such radiation and thus the radiation pattern from this acoustically reflective lens will be somewhat incoherent and may exhibit interference due to this constraint.
U.S. Pat. No. 4,836,328 to Ferralli, provides a geometrically shaped reflective lens for a transduction element such that all acoustic path lengths from the transduction element surface to the lens focal element are substantially identical. A geometrically shaped reflective lens, is also provided, which will focus acoustic waves produced by the transduction element to a focal element which is characteristic of the lens, as well as increase the beamwidth of the acoustic radiation emanating from the lens and provide for the relative consistency of the beamwidth as a function of acoustic wave frequency. The Ferralli patent utilizes a multiple transducer and an elliptical shaped back box. The transducer cone is placed forward into the apex of the ellipsoid and is subject to comb filtering. The Ferralli also does not provide a complex baffling and wave guide system.
This invention utilizes multiple transducers and places the transducer cone forward into the apex of the ellipsoid and is subject to comb filtering. This invention does not use a complex baffling and wave guide system and has not been tested free field.
None of the prior art patents have, however, overcome the problem of diverse bass waves. The instant invention can be produced with the elliptical shaped backbox which is a common design, however, all the other complex baffling and wave guides must be employed in order to achieve directionality under 90 cycles. The increased directionality provided by the instant device promotes cleaner sound with less room reverberation and low frequency feedback than omni-directional oriented bass enclosure designs due to the popular theoretical misconception that bass waves can not be made directional under 90 cycles.